That is not entirely correct. Muhammad's form of warfare is only applicable during Muhammad's time. However the principle is the same. All non combatants (including women, children, old people and people who takes sanctuary in places of worship) are never to be harmed. However how one fight should not be strictly medieval. It's idiotic.

The core of islam must never be change! Your understanding of the core of islam seems to extend into the tradition and culture of race/ethnics.
Some ethnics' tradition change their interpretation of the teachings of islam but that does not mean the core is changed. Overtime some of their traditions became synonym with islam that they thought that it is a must.
For example in Malaysia Malay weddings have adat bersanding and malam berinai. At the end of the 80's these traditions were still thought to be islam teachings but in reality it is just a Malay tradition based from the Indian wedding.

Core Islamic Beliefs: Six Articles of Faith

More than a billion Muslims share a common set of fundamental beliefs that are described as "Articles of Faith." These articles of faith form the foundation of Islamic belief system.

1.  Belief in One God: The most important teaching of Islam is that only God is to be served and worshipped.  Also, the biggest sin in Islam is to worship other beings with God.  In fact, Muslims believe that it is the only sin that God does not forgive if a person dies before repenting from it.

2.  Belief in Angels: God created unseen beings called angels who work tirelessly to administer His kingdom in full obedience.  The angels surround us at all times, each has a duty; some record our words and deeds.

3.    Belief in Prophets of God: Muslims believe that God communicates His guidance through human prophets sent to every nation.  These prophets start with Adam and include Noah, Abraham, Moses, Jesus and Muhammad, peace be upon them.  The main message of all the prophets has always been that there is only One true God and He alone is worthy of being supplicated to and worshipped.

4.    Belief in Revealed Books of God: Muslims believe that God revealed His wisdom and instructions through ‘books’ to some of the prophets like the Psalms, Torah, and the Gospel.  Over time, however, the original teachings of these books got distorted or lost.  Muslims believe the Quran is God’s final revelation revealed to Prophet Muhammad and has been fully preserved.

5.    Belief in Day of Judgment: The life of this world and all that is in it will come to an end on an appointed day.  At that time, every person will be raised from the dead.  God will judge each person individually, according to his faith and his good and bad actions.  God will show mercy and fairness in judgment.  According to Islamic teachings, those who believe in God and perform good deeds will be eternally rewarded in Heaven.  Those who reject faith in God will be eternally punished in the fire of Hell.

6.    Belief in Destiny and Divine Decree: Muslims believe that since God is the Sustainer of all life, nothing happens except by His Will and with His full knowledge.  This belief does not contradict the idea of free will.  God does not force us, our choices are known to God beforehand because His knowledge is complete.  This recognition helps the believer through difficulties and hardships.

Core Religious Practice of Islam: The Five "Pillars" of Islam

In Islam, worship is part of daily life and is not limited to mere rituals.  The formal acts of worship are known as the five "pillars" of Islam.  The five pillars of Islam are the declaration of faith, prayer, fasting, charity, and pilgrimage.

1.    Declaration of Faith: The "Declaration of Faith" is the statement, "La ilaha illa Allah wa Muhammad Rasul-ullah", meaning "There is no deity worthy of being worship except God (Allah), and Muhammad is the Messenger (Prophet) of God").  The Declaration of Faith is more than just a statement; it must be shown with one's actions.  To convert to the faith of Islam, a person has to say this statement.

2.    Daily Prayer: Prayer is a method by which a Muslim connects to God and gathers spiritual strength and peace of mind.  Muslims perform five formal prayers a day.

3.    Zakah: A type of charity.  Muslims recognize that all wealth is a blessing from God, and certain responsibilities are required in return.  In Islam, it is the duty of the wealthy to help the poor and needy.

4.    Fast of Ramadan: Once each year, Muslims are commanded to fast for an entire month from dawn to sunset.  The period of intense spiritual devotion is known as the fast of Ramadan in which no food, drink and sex is allowed during the fast.  After sunset one can enjoy these things.  During this month Muslims practice self-control and focus on prayers and devotion.  During the fast, Muslims learn to sympathize with those in the world who have little to eat.

5.    The Hajj Pilgrimage to Mecca: Every Muslim strives to make once-in-a-lifetime pilgrimage to the sacred sites in Mecca, in present-day Saudi Arabia.  It is the most intense spiritual experience for a Muslim.  Typically, 2-3 million perform hajj every year.''''

http://www.islamreligion.com/articles/10256/